The effects of both acute and chronic occupational exposures have been shown at N₂O air concentration levels as low as 50 parts per million (ppm), resulting in bone marrow depression (primarily granulocytopenia), paresthesias, difficulty concentrating, equilibrium disruption and impaired visual effects.²¹ Results from recent studies show chronic N₂O exposure is associated with alterations in vitamin B12 and plasma homocysteine concentrations.^22,23 Sanders and colleagues²⁴ recently conducted the most comprehensive review of the biological effects of N₂O. The review covers the mechanical and toxicological effects of N₂O in patients and how to minimize the potential toxicity to medical and dental care providers.

In response to these early findings and in an effort to effectively control exposures with potentially adverse effects, several guidelines have been published that define appropriate use and control criteria for N₂O. In 1977, NIOSH published a technical report, in which it identified the use of scavenging systems as a key engineering control and, on the basis of this new technical feasibility, recommended controlling waste N₂O to less than 25 ppm during administration.^25,26 These recommendations are based on the open system as practiced in dental operatories versus the closed system as practiced in hospital operating rooms (that is, an endotracheal tube with its balloon). In 1980, the ADA began recommending use of scavenging devices during N₂O delivery and the creation of occupational exposure monitoring programs.¹⁴ McGlothlin and colleagues²⁷ reported that N₂O emissions were inconsistently controlled below the 25 ppm value, even with the use of scavenging systems. In 1977, NIOSH established a recommended exposure limit (REL) of less than 25 ppm during the time of administration.²⁵ NIOSH followed this with a second technical report that described the factors that most effectively control exposure to waste N₂O.¹⁸ Crouch and colleagues²⁸ concluded that the 1977 NIOSH REL could be achieved by using a scavenging system with proper equipment and system maintenance; best work practices; and good-quality, well-designed local and general ventilation systems.

In 1997, the ADA Council on Scientific Affairs reviewed all available literature to make its own recommendation regarding appropriate and acceptable exposure levels. It did not recommend an occupational exposure level but instead deferred to existing limits established by NIOSH and ACGIH.²⁹ It did, however, publish its own guidelines regarding appropriate use of N₂O sedation in March 1997.³⁰ The ADA made 10 recommendations that address the use of appropriate engineering controls (that is, scavenging equipment): using the vacuum scavenger at 45 liters per minute (lpm), ventilating gases to the outside, ensuring good room air exchange rates, inspecting N₂O lines, checking mask and hoses for leaks, ensuring proper mask fit, minimizing mouth breathing, checking reservoir bag for flow, following N₂O delivery with 100 percent oxygen and monitoring personnel periodically for occupational exposure.

The effects of both acute and chronic occupational exposures have been shown at nitrous oxide air concentration levels as low as 50 parts per million.

OSHA also has published guidelines regarding the use of anesthetic gases.^31,32 OSHA’s recommendations resemble those of NIOSH and ADA. However, one set of guidelines³² points to location-specific controls for dental operatories similar to ADA recommendations. Although it does not specifically regulate N₂O, OSHA expects reasonable efforts to be made to adequately control occupational exposure to N₂O.

We conducted a study to determine the effectiveness of two waste anesthetic gas scavenging systems of divergent designs (system II had been used widely for several years, and system I was new to the market) at reducing occupational exposure to N₂O in dentistry. We compared the results we obtained from the scavenging systems with each other to established occupational exposure levels. We also attempted to understand how work practices might affect the effectiveness of each system.

Our study focused on the performance and work practices of the two N₂O scavenging systems. We used infrared imaging to evaluate the fugitive emissions of N₂O during dental surgeries, and we used visual video imaging to evaluate work practices. In addition, we used a real-time sampling instrument to document N₂O concentrations during the dental surgeries.

	METHODS

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The institutional review board at Purdue University approved our study design and consisted of three distinct phases (institutional review board protocol reference no. 0607004186, approval start date Aug. 8, 2006). In phase I, we collected data from oral surgeries in which surgeon 1 (J.E.M.) used the Safe Sedate Dental Mask (Airgas, Radnor, Pa.) system (system I) to administer and evacuate waste anesthetic gas. In phase II, we replaced system I with the Porter Nitrous Oxide Sedation System (Porter Instrument, Hatfield, Pa.) (system II); the same oral surgeon as in phase I performed all of the surgeries. In phase III, system I was used for each surgical event, and a second oral surgeon designated as "inexperienced" with this particular system performed all surgeries. Surgeon 2 (M.B.) was employed at a different practice site than surgeon 1, and he and his staff members had no previous experience with this system. Because system I was new, surgeon 2 received minimal training from a company sales representative on how to operate it. We designed phase III to reflect an actual system’s performance in controlling occupational exposure to N₂O in a typical, new-user scenario. This phase also allowed us to compare work practices between two users with varying levels of experience and motivation for appropriate system use.

We designed the phases of our study to control for two key independent variables—scavenging system design and user experience with a given system—and to understand what effects these variables have on occupational exposure to N₂O in dental operatories.

System I was developed to improve the patient’s comfort during anesthetic gas delivery, eliminate usability issues in the clinical environment associated with the available scavenging systems and reduce practitioners’ and staff members’ occupational exposure to waste anesthetic gases. It is a fully disposable system that delivers the anesthetic gas directly into the nares via adjustable canulae that pass through the mask. The waste anesthetic gas is scavenged from the mask via a single hose linked from the mask to a vacuum system. Additional perforations at the base of the mask allow users to insert a canula for monitoring carbon dioxide levels if necessary.

System II is one of the most commonly used scavenging systems,¹² and it relies on a double-domed design to deliver anesthetic gas and remove excess gas during use. The inner mask is connected to the anesthetic gas source and delivers the gas to the patient by flooding the enclosed nasal area. The outer mask is the extraction element that removes excess and nasally expired gases.³³

The oral surgery practices of surgeon 1 and surgeon 2 were two data collection sites for our study. Surgeon 1’s practice consisted of seven oral surgery operatories, patient waiting and recovery areas, and office space for staff members. It used a common heating, ventilation and air conditioning system to supply air to and return air from each of the rooms. The operatory used during this study was 10.67 feet long by 10.00 feet wide by 10.00 feet high and had five calculated air changes per hour. The operatory maintained negative air pressure relative to the adjacent hallway.

Surgeon 2’s practice consists of five operatories, common office areas and patient waiting and recovery rooms. It also used a common heating, ventilation and air conditioning system to maintain adequate ventilation to each of the rooms. We limited data collection to a single operatory that was 9.50 feet long by 11.92 feet wide by 9.00 feet high with six calculated air changes per hour. The operatory also maintained negative air pressure relative to the adjacent hallway.

We obtained all airflow measurements by means of an airflow measurement hood (Balometer Capture Hood, Alnor, Costa Mesa, Calif.) during the data collection at each facility.

We collected three types of data for each of the surgeries: infrared thermography by means of an infrared camera (Merlin Mid InSb Midwave Infrared Camera, FLIR Systems, Boston), digital videography by means of a camcorder (HandyCam, DCR-SR100, Sony, Tokyo) and real-time N₂O air concentration levels by means of an infrared spectrophotometer (MIRAN 1B SapphIRe Ambient Air Analyzer, Thermo Fisher Scientific, Waltham, Mass.). To facilitate data analysis, we synchronized date and time for all instruments, thus enabling real-time N₂O air concentration levels to be matched with corresponding infrared and video images. This synchronization aided us in our understanding of how a particular task element contributed to occupational exposures to N₂O.

All dental surgeries for which we obtained data were standardized with regard to vacuum flow rates at 45 lpm using flowmeters (Dwyer, Upper Saddle River, N.J.). We minimized study variability by collecting all data on third-molar extraction surgeries. We included in the study only surgeries in which at least two teeth were extracted. Typical conscious sedation with N₂O anesthesia ranged from approximately 30 to 60 minutes. Each dental patient underwent intravenous sedation in which N₂O was administrated at 60 percent and oxygen at 40 percent with 5 lpm of airflow (3 lpm N₂O, 2 lpm oxygen).

We collected data Dec. 18, 2006, through Dec. 20, 2006, for phase I; Dec. 20, 2006, through Dec. 22, 2006, for phase II; and Jan. 3, 2007, through Jan. 5, 2007, and Jan. 8, 2007, for phase III.

We completed all statistical analyses by means of commercially available software (Excel 2003, Microsoft, Redmond, Wash.) by using the data analysis function or by means of other statistical software (Minitab, Release 14.20 Statistical Software, Minitab, State College, Pa.). We used the Excel spreadsheet to transform the N₂O air concentration data from the text files created with instrument uploads. Within the spreadsheet, we parsed the data into values for individual surgeries and completed descriptive statistical analyses. The descriptive statistics allowed us to compare the study data with relevant exposure guidelines. We used the Minitab software to complete the one-way analysis of variance to analyze the differences in data between each of the study phases. A statistician from the Purdue University Statistical Consulting Group (West Lafayette, Ind.) oversaw each stage of data analysis to ensure that data were handled appropriately and that we applied relevant statistical analyses to the study data.

	RESULTS

TOP ABSTRACT METHODS RESULTS DISCUSSION CONCLUSIONS References

 
We collected a minimum of 13 data sets in each phase of the study. In phase I, 16 surgeries met all study criteria for acceptance. In phase II, 14 surgeries met the study criteria. Of the 15 surgeries we evaluated in phase III, we excluded two, as they were not representative owing to a high starting N₂O air concentration level in the operatory before the beginning of the surgery. This high N₂O air concentration level was attributed to use of N₂O in the two immediately adjacent operatories by other surgeons who were not involved in the study. Phase III nevertheless had the minimum required number of data sets as determined by means of power calculations for statistical significance.

In phase I, surgeon 1, who was experienced in its use, used system I in an oral surgery office. We collected data for the 16 surgeries across three days; no other dental surgeries were performed in any other operatory in this facility during the collection period. From the N₂O air concentration data logged, we selected the subset of values that corresponded from when the N₂O was turned on to when it was turned off. On the basis of these values and the surgery duration, we determined an average surgical N₂O air concentration level, which allowed us to compare it with the NIOSH REL of 25 ppm or less. Table 1 shows the duration and the average N₂O air concentration level during each phase I surgery.

We used infrared thermography to conduct a qualitative evaluation of the performance of the scavenging systems and the study variables in each phase. Although there were several task elements, each surgery could be separated into two basic activity groups: surgical and nonsurgical. We defined surgical activity as the active portions of the surgery (that is, surgeon present and engaged in either the extraction of the tooth or the administration of local anesthetic). Nonsurgical activity included times when the surgeon was not actively performing procedures. Figures 1 and 2 (page 197) show typical time and procedure according to infrared thermographic images from each of the phases during surgical and nonsurgical events. The images are typical of each phase, and we selected them for their similarity with regard to time during surgery, extraction element and patient’s activity according to data collection notes and review of digital videography (for example, talking, breathing rate and anxiety).

View larger version (55K): [in this window] [in a new window]   Figure 1. Infrared thermographic images from each of the phases during nonsurgical events. The boxes delineated by the yellow rules define the primary portion of visible nitrous oxide emissions. The corresponding nitrous oxide air concentration levels are 47.3 parts per million (ppm) for phase I (A), 219.1 ppm for phase II (B) and 120.8 ppm for phase III (C).

View larger version (56K): [in this window] [in a new window]   Figure 2. Infrared thermographic images from each of the phases during first extraction surgical events. The boxes delineated by the yellow rules define the primary portion of visible nitrous oxide emissions. The corresponding nitrous oxide air concentration levels are 194.9 parts per million (ppm) for phase I (A), 471.7 ppm for phase II (B) and 200.1 ppm for phase III (C).

The work practice evaluation process involved a qualitative review of the N₂O air concentration data from phases I and III (the experienced user versus the inexperienced user) to identify patterns in or parts of the surgery that seemed to be predictors of system I’s ability to perform most effectively. The results of the data analysis showed a statistically significant reduction (P .02) in occupational exposure to N₂O in the office with the experienced user compared with that in the office with the inexperienced user. Evaluation of the data from phase I suggested that N₂O air concentration levels obtained five minutes after the beginning of N₂O delivery generally indicates whether average N₂O air concentration levels will be high.

When good work practices were absent, N₂O air concentration levels were more likely to increase. Three key work practices that occurred in phase I and did not occur in phase III minimized N₂O air concentration levels from the onset of N₂O use, which decreased overall averages of airborne concentration levels of N₂O. These key work practices were ensuring a proper fit of the scavenging system by tightening the elastic straps of the dental mask so that it was placed snugly against the patient’s face and did not move, turning on vacuum exhaust before beginning the administration of N₂O and discouraging patient’s talking and mouth breathing. Ensuring that the proper mask size is selected was a fourth work practice that also should be considered key. It was followed appropriately in phases I and III.

	DISCUSSION

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On the basis of the study design, the NIOSH REL of 25 ppm or less during the time of administration is the most appropriate occupational exposure limit for N₂O with which the N₂O air concentration data should be compared. The ability to achieve this value with statistical certainty was not demonstrated during any of the three study phases. None of the surgeries in any of the three study phases had an average N₂O air concentration level of less than 25 ppm during the time of administration.

The inability of any treatment group to meet the NIOSH REL is inconsistent with data in the published literature. Donaldson and Orr¹³ and Certosimo and colleagues¹² reported study N₂O air concentration levels of less than 50 ppm with system II. In the study by Certosimo and colleagues,¹² simulated rather than actual surgical procedures were being performed during the N₂O testing session, which meant that N₂O leakage was controlled more easily.

Crouch and colleagues²⁸ indicated that when system II was combined with an auxiliary exhaust device it met the REL of 25 ppm or less in 60 percent of the evaluated cases. The mean N₂O air concentration level achieved with system II in phase II of our study—225.6 ppm—far exceeded previously reported values despite the fact that the surgeon in phase II was experienced with the system and followed the four key work practices.

Comparing our study’s use of system I with information in the published literature was not possible because this is the first independent study in which the system’s effectiveness has been evaluated. The data, however, suggest that the system performs as well as or better than system II. There is merit in further research to evaluate the system’s effectiveness under additional study conditions and variables.

The dramatic change in system I’s performance between phases I and III is an indication of the effect of work practices on the effectiveness of any scavenging system, as suggested by Crouch and colleagues²⁸ and McGlothlin and colleagues.²¹ As a function of experience and system knowledge, surgeon 1 had firmly established key work practices within his office, which likely accounts for the difference in results between surgeons 1 and 2 when they used the same system. The minimal training provided by a sales representative to surgeon 2 may be typical of that provided to any new user and did not highlight key work practices, leaving surgeon 2 to learn from experience. The difference in results reinforces the contention that work practices play a major role in the success or failure of any scavenging system to control occupational exposures.

The infrared thermographic data consistently indicated that the visible emissions seen with system II generally were greater than those seen with system I, even in the absence of any patient-provider interaction. The corresponding N₂O air concentration data also demonstrated that there were greater N₂O air concentration levels with system II in the absence of any patient-provider interaction.

The evaluation of work practices associated with the first five minutes of N₂O delivery confirmed that implementation of key work practices is essential to maximize the efficiency of any scavenging system. Adverse effects resulting from a poorly fitting mask, patient’s talking and improper use of the vacuum system were evident in the infrared thermographic images and the corresponding increases in N₂O air concentration levels that we saw when poor work practices occurred. Key work practices do not have to be elaborate, difficult or resource-consuming to have a dramatic effect. However, they must be used systematically. Therefore, dentists and their staff members must be trained regarding key work practices, understand the reasons for implementing them and apply them consistently. Future evaluations of scavenging systems should explore in more depth the effect of work practices. After such evaluations, scavenging systems with the best demonstrated technical feasibility combined with a set of key work practices can be used to reduce the occupational exposures to N₂O.

Our study is different from previous research. System I had not been evaluated independently with regard to its ability to reduce occupational exposures to N₂O relative to established guidelines or to any other system. An informal evaluation of the system by the developer suggested that this system was highly effective in reducing occupational exposure to waste N₂O. In this study, we sought to verify this finding independently.

Our study differs from traditional scavenging system studies in that we aimed to demonstrate that the execution of key work practices can have a substantial effect on airborne concentrations of N₂O. By determining the practices that most effectively reduce occupational exposure, scavenging-system users can combine well-designed systems with improved work practices to achieve the greatest possible reduction in levels of exposure to waste N₂O.

We used state-of-the-art tools to accomplish our goals. These tools included an infrared imaging camera, a digital video camera, and a real-time airborne sampling instrument to detect and monitor N₂O concentrations. These tools helped us depict the N₂O fugitive emissions and detect changes in N₂O concentrations as a function of work practices during the dental surgical procedure.

	CONCLUSIONS

TOP ABSTRACT METHODS RESULTS DISCUSSION CONCLUSIONS References

 
The results of our study’s comparison of the performance of system I with that of system II showed that system I performed significantly better than did system II in reducing occupational exposure to waste anesthetic gas emissions of less than the ACGIH threshold limit value of 50 ppm during an eight-hour day.

In addition, the use of key work practices improved system I’s ability to reduce occupational exposure to N₂O effectively toward the NIOSH REL of 25 ppm or less during the time of administration. The results of our qualitative evaluation of infrared thermography and N₂O air concentration data, as well as the systematic evaluation and documentation of best work practices, suggest that appropriate mask size selection and mask adjustment, minimal talking and mouth breathing by the patient, and use of a vacuum scavenging system are key work practices that must be implemented appropriately to ensure maximum system effectiveness.